Feature: Linux users could not run Mu using Flatpak

Solution: Add a manifest file for flatpak-builder

Known issues:
- Icon does not display correctly on my KDE desktop
- Mu does not detect device vendor and properties, failing to connect via serial (overriding the code makes it work).

.gitignore

@@ -24,6 +24,7 @@ var/
 *.egg-info/
 .installed.cfg
 *.egg
+.flatpak-builder
 
 # Bluemix vendored packages
 vendor/

flatpak/mu.codewith.Editor.appdata.xml

@@ -0,0 +1,42 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+	<id>mu.codewith.Editor</id>
+	<metadata_license>CC-BY-SA</metadata_license>
+	<name>Mu Editor</name>
+	<summary>A simple editor for beginner Python programmers</summary>
+	<description>
+        <p>
+            Mu is a simple code editor for beginner programmers based on extensive feedback from teachers and learners.
+            Having said that, Mu is for anyone who wants to use a simple "no frills" editor.
+        </p>
+        <p>Mu is a modal editor with modes for many different ways to use Python to create cool and interesting things.</p>
+        <p>
+            Mu is written in Python and works on Windows, macOS, Linux and Raspberry Pi. The project's public facing website is https://codewith.mu/.
+            We celebrate the work done by users of mu at https://madewith.mu/.
+        </p>
+	</description>
+	<url type="homepage">https://codewith.mu/</url>
+	<url type="bugtracker">https://github.com/mu-editor/mu/issues/</url>
+	<url type="help">https://codewith.mu/en/howto/1.1/</url>
+	<launchable type="desktop-id">mu.codewith.Editor.desktop</launchable>
+	<releases>
+		<release version="1.1.1" date="2022-02-22">
+			<description>
+				<p>
+                    This is the first stable release of the 1.1.* version of Mu.
+                </p>
+			</description>
+		</release>
+	</releases>
+	<project_license>GPL-3.0-or-later</project_license>
+	<developer_name>Nicholas H.Tollervey</developer_name>
+	<screenshots>
+		<screenshot type="default">
+			<image type="source">https://codewith.mu/img/en/tutorials/mu_ui.png</image>
+		</screenshot>
+		<screenshot>
+			<image type="source">https://codewith.mu/img/en/tutorials/mode_selector1-1.png</image>
+		</screenshot>
+	</screenshots>
+	<content_rating type="oars-1.1"/>
+</component>

flatpak/mu.codewith.Editor.desktop

@@ -0,0 +1,10 @@
+[Desktop Entry]
+Name=Mu Editor
+GenericName=A simple editor for beginner Python programmers
+Exec=/app/python-build-standalone/install/bin/mu-editor
+Terminal=false
+Icon=mu.codewith.Editor
+Type=Application
+Categories=Development;IDE;
+X-Desktop-File-Install-Version=1.1.1
+StartupWMClass=mu-editor

flatpak/mu.codewith.Editor.yml

@@ -0,0 +1,74 @@
+# Build using `flatpak-builder --user --install --force-clean build-dir mu.codewith.Editor.yml`.
+# Then run using `flatpak run mu.codewith.Editor`.
+
+app-id: mu.codewith.Editor
+
+runtime: org.freedesktop.Platform
+runtime-version: '21.08'
+sdk: org.freedesktop.Sdk
+
+command: /app/python-build-standalone/install/bin/mu-editor
+
+finish-args:
+  - --filesystem=home
+  # X11/Wayland
+  - --share=ipc
+  - --socket=x11
+  - --socket=wayland
+  - --device=dri
+  - --socket=pulseaudio
+  # Network access for Mu users
+  - --share=network
+  # MicroPython devices connect over USB
+  - --device=all
+
+modules:
+  # Use python-build-standalone since this is what `pup` uses for Windows and macOS.
+  # This should make the builds more homogenous across platforms.
+  - name: python-build-standalone
+    buildsystem: simple
+    build-options:
+      build-args:
+        - --share=network
+    build-commands:
+      - cp -pr . /app/python-build-standalone
+    sources:
+      - type: archive
+        url:  https://github.com/indygreg/python-build-standalone/releases/download/20220630/cpython-3.8.13+20220630-x86_64-unknown-linux-gnu-lto-full.tar.zst
+        sha256: 217fd3deccdc59e074800a2d90ea9176395d1630e1e24f84c062e6d5bf3e03a3
+
+  - name: mu
+    buildsystem: simple
+    build-options:
+      build-args:
+        - --share=network
+    build-commands:
+      - /app/python-build-standalone/install/bin/pip install --upgrade pip
+      - /app/python-build-standalone/install/bin/pip install .
+      - /app/python-build-standalone/install/bin/pip install pyserial
+      # Prefetch wheels used in the user virtual environment.
+      # Changes directory to `cd /tmp` in order to avoid importing `mu.wheels`
+      # from the git repository.
+      - cd /tmp && /app/python-build-standalone/install/bin/python3 -m mu.wheels
+    sources:
+      - type: archive
+        url: https://github.com/mu-editor/mu/archive/refs/tags/v1.1.1.tar.gz
+        sha256: 6ea06d09ba0ed15a2bdd87b62ad1c18a0b1edc7000956209720fcc4ad290458e
+
+  - name: desktop-integration
+    buildsystem: simple
+    build-options:
+      build-args:
+        - --share=network
+    build-commands:
+      - install -Dm644 -t /app/share/applications mu.codewith.Editor.desktop
+      - install -Dm644 -t /app/share/metainfo mu.codewith.Editor.appdata.xml
+      - install -Dm644 -t /app/share/icons/hicolor/scalable/apps mu.codewith.Editor.svg
+    sources:
+      - type: file
+        path: mu.codewith.Editor.desktop
+      - type: file
+        path: mu.codewith.Editor.appdata.xml
+      - type: file
+        path: mu.codewith.Editor.svg
+